Clarify how service_account in toml is used

What does this MR do?

Per https://gitlab.com/gitlab-org/gitlab-runner/-/blob/master/common/config.go#L259. Also supported by https://gitlab.com/gitlab-org/gitlab-runner/-/blob/master/executors/kubernetes/kubernetes.go#L968 and https://gitlab.com/gitlab-org/gitlab-runner/-/blob/master/executors/kubernetes/overwrites.go#L142 as well as supporting test at https://gitlab.com/gitlab-org/gitlab-runner/-/blob/master/executors/kubernetes/kubernetes_test.go#L1278

Related issues

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